(a) Field of the Invention
This invention relates to paper machine clothing such as forming fabrics, press felts and dryer fabrics which are woven of plastic polymeric yarns. More particularly the invention relates to providing a fabric having superior wear resistance.
(b) Description of Prior Art
Paper machine clothing is provided in the form of endless belts which run over supporting rolls and carry the web of wet pulp or paper through the machine. For example, an aqueous suspension of cellulose fibers, comprising one part or less fibers in 99 parts or more of water by weight, is flowed on to an endless rotating fabric. This fabric may be woven of multifilament or monofilament synthetic strands, in single layer or in multi-layer structures, that provide support for the fibers of pulp and at the same time uniform and adequate drainage. As this belt, or forming fabric, as it is called, passes over water extraction devices such as table rolls, drainage foils and suction boxes, the water content of the suspension supported on the fabric is reduced to about 80 to 85 percent.
The thin web of fibers, now self supporting, is removed from the forming fabric and passes to a series of one or more press sections where it is deposited on other endless belts of relatively thick fabric, one or both surfaces of which may be composed of a batt of synthetic or natural fibers needled to a backing of woven plastic yarn. These press felts, as they are called, carry the web of paper through the nips of press rolls where more of the water remaining in the web is squeezed into the absorbent felts until the water content is lowered to about 60%. At this point it is not generally practical to attempt further water removal by direct extraction such as with vacuum or pressure.
The web of paper is then passed to the dryer section of the machine where the remainder of the water is removed by an evaporation process accelerated by the application of heat. In the dryer section there are a number of large, hollow cast iron or steel cylinders over which the paper web passes in serpentine fashion. The cylinders are rotated synchronously to facilitate the passage of the web and are heated by steam condensing within. The web of paper is held in intimate contact with portions of the heated surfaces of the cylinders by dryer fabrics. Dryer fabrics may be woven of synthetic yarns in simple or in very complex weaves generally in two or more layers to form a comparatively impermeable fabric.
For each of these uses the paper machine fabrics must be structurally stable in the plane of the cloth, flexible in at least the machine direction and have sufficient tensile strength in the machine direction to resist stretching. They must also be reasonably resistant to corrosion and the absorption of moisture and possess a surface that is resistant to abrasive wear. Abrasive wear plays an important role in determining the useful life of all paper machine clothing and this is most apparent in forming fabrics which, because they must travel in contact with various stationary dewatering elements, are the most susceptible to abrasive wear. In addition, forming fabrics are designed to remove large amounts of water while retaining the fibers in the paper web, thus they are very fine and have relatively small volumes of material available for abrasive wear. For this reason the present invention is directed particularly to forming fabrics although it will be applicable to other paper machine clothing, as well, where wear is also a factor.
In the case of forming fabrics, for example, the design of the weave pattern may be selected so as to provide a prominent knuckle in the cross-machine direction yarn, usually the weft, and at the wear side of the fabric that is presented to those elements of the paper machine upon which the fabric is in contact as it runs. In this way, the more prominent weft knuckles absorb the greater portion of the wear while the warp yarns, which lie substantially within the fabric, are protected from wear and maintain the strength of the fabric throughout its life span. This concept is known in the art and is, in fact, described in U.S. Pat. No. 4,184,519 issued to P. L. McDonald et al. and U.S. Pat. No. 4,281,688 issued to B. W. Kelly et al. both related to single layer forming fabric and in U.S. Pat. No. 4,041,989 issued to A. B. Johansson et al., related to double layer forming fabric. The use of wear resistant materials in forming fabrics is described in U.S. Pat. No. 4,289,173 issued to Terence Miller.